1. Field of The Invention
The present invention is related generally to a microwave transponder. More specifically, the present invention pertains to a microwave transponder having a circular window structure that fulfills impedance matching in order to transfer maximum energy.
2. Description of The Related Art
A microwave transponder is utilized in various applications. For example, a radio frequency identification (RFID) system, which is one of the various applications, is coming into increasing use in industry.
FIG. 1 is a schematic block diagram of a conventional RFID system. Referring to FIG. 1, the RFID system includes a transponder 10, a reader 20, and a main computer 30. The reader 20 includes a transmitter and a receiver. The reader 20 transmits an interrogation signal to the transponder 10 through an antenna installed in the reader 20 (hereinafter, refer to as a reader antenna), and receives necessary data from the transponder 10. The transponder 10 is classed into an active type including a battery and a passive type supplied power from the reader 20. The active type, which includes an energy source, provides a long communication distance, but is large-sized and inoperative in conditions when it is hard to obtain the power source. The passive type, which derives energy from the reader 20, is compact-sized and free from the power, but provides a shorter communication distance than the active type.
The transponder 10, which is the passive type, gets all the needed energy from the interrogation signal of the reader 20. The transponder 10 responds by sending its data to the reader 20 by backscatter communication using the received energy. The reader 20 wirelessly identifies the transponder 10 by reading the data.
FIG. 2 is a schematic view of a conventional transponder 8 disclosed in the United States Patent Application No. 2003/0063002 A1. Referring to FIG. 2, the transponder 8 includes a thin metal plate 2 forming an antenna, a semiconductor chip 3 mounted within or near a terminal gap 4, and a narrow long slit 5 extended from the terminal gap 4 toward an outer edge of the metal plate 2. The slit 5 mainly matches an impedance of the semiconductor chip 3 with that of the antenna.
Recently, it has been required that a transponder be adhered onto a curved installation such as a cylindrical or spherical surface, or in the vicinity of a corner of an installation. When the transponder is bendable, the antenna on a flexible substrate is adversely affected by various distortions such as bending. The impedance of the semiconductor chip is not significantly affected by the deformation of the transponder while the impedance of the antenna is obviously affected by the deformation of the transponder. Accordingly, the impedances of the antenna and the semiconductor chip mismatch, thus limiting an operating range of the identification. Specifically, if the transponder is constructed to adhere on a curved installation, its operating range is limited so as to depreciate the performance or results in standstill.
Conversely, when a larger operating range is needed, the microwave power radiated by the reader has to be increased. However, the increase of the microwave power is restricted due to safety regulation or other legislation.